Elevated requirements are placed upon films for packaging applications with regard to mechanical strength and optical properties, namely transparency. There is thus a constant effort to ensure optimum transparency. The transparency of composite films based on polyamides and polyolefins is elevated if the generally more turbid polyolefin layers may be made as transparent as possible. The emphasis should thus be placed on the transparency of the polyolefin heat sealing layer. According to the prior art, it is known that processing polyolefins using the blown film process gives rise to relatively turbid films, whereas material processed using the flat film process gives rise to crystal clear, transparent films (Hagen, H., Domininghaus, H.: Polyethylen und andere Polyolefine, Verlag Brunke Garrels, Hamburg 1961, pp. 175 et seq.). This applies in particular also to polypropylene. However, since blown films have other attractive properties, efforts are being made to invent film formulations which improve transparency, irrespective of the production process.
It has been suggested to achieve this by a polymer blend prepared from PP and LLDPE. Film formulations are known from the patent literature, which, in order to achieve this object, contain above 50 wt. % of LLDPE and are thus by their nature more LLDPE films than PP films (JP 5 8217 534, EP 350 859, EP 423 387, U.S. Pat. No. 5,139,855, EP 0 350 859).
It has also been found that mixtures prepared from PP and LLDPE, which predominantly contain PP (JP 5 7159 832, JP 01152 144, WO 9214784) yield transparent films. A feature common to all these developments is that 20 wt. % or above of LLDPE are required to achieve this object.
EP-397 517 A also describes a polymer blend prepared from PP and LLDPE which contains 0.5 to 20 wt. % of LLDPE. According to the prior art it is clearly possible to incorporate such small quantities of LLDPE into PP only if the PP and the LLDPE are as similar as possible. Such similarity is defmed by density. The density of PP is approximately 900 to 910 kg/m.sup.3, that of the LLDPE, which is described as a blend component, is in the range from 860 to 910 kg/m.sup.3. An LLDPE of such a low density is also known as VLDPE (very low density polyethylene).
The object thus arises of providing a film containing polypropylene which may be produced using the blown film or flat film process. The decisive properties of the film to be provided comprise elevated transparency with simultaneously high mechanical strength. The film may be both a single layer film and a multi-layer film.